It is known that ultraviolet rays (UV) induce acute skin reactions such as erythema, skin aging, and skin cancer. The ultraviolet rays contained in sunlight are classified, based on the wavelength, into three types: UV-A (320 nm-400 nm), UV-B (280 nm-320 nm) and UV-C (200-280 nm). Among them, UV rays having an effect on living bodies are UV-A and UV-B; whereas UV-C is harmless since it normally cannot pass through the atmosphere.
It is known that UV-B is a main cause of sunburn in the outdoors and has relatively larger energy than UV-A. When UV-B is absorbed by skin layer, it reaches the stratum corneum and the epidermis, and causes acute skin pigmentation such as spots and freckles. UV-B is also known to cause immunosuppression involved in aging and onset of skin cancer.
UV-A has a longer wavelength than UV-B and the energy is lower; however, UV-A penetrates into a further deeper portion of the skin than UV-B and is known to reach the dermis. As a result, not only acute skin pigmentation such as spots and freckles but also reduction in elasticity of the dermis (actinic elastosis) is caused, with the result that early skin aging such as wrinkles and sagging is induced. Furthermore, in recent years, it has been found that UV-A also causes immunosuppression and is involved in precancerous skin lesion and onset of skin cancer.
UV-B varies in amount depending upon e.g., season, weather and latitude; whereas the amount of UV-A reached the surface of the earth is constant throughout the year. Therefore, it is important to protect the skin also from UV-A.
UV protectors currently available in the market are divided into UV-absorbing agents and UV scattering agents. The UV-absorbing agents convert the UV energy into heat energy and release it, and include, for example, an organic compound such as 4-tert-butyl-4′-methoxydibenzoylmethane. The UV scattering agents contain an inorganic particle such as titanium oxide (TiO2) and zinc oxide (ZnO), and when the UV scattering agent is applied to the skin, the inorganic particles present on the skin reflect UV rays and serve as a UV barrier.
The UV-absorbing agents have problems: (1) the UV-absorbing agents are easily decomposed by light and have poor stability, (2) molecular excitation occurs and accelerates production of melanin to cause itchiness and allergy, and (3) a bad image is given to the users due to chemically synthesized substances. The UV scattering agents have the problems: (1) skin likely looks white when applied and the user tends to feel heavy in the skin, (2) generation of active oxygen is induced, and (3) pores of skin are closed and skin respiration may be inhibited. Because of these problems, development of a naturally derived safe UV-absorbing substance has been highly expected.
Mycosporine-like amino acids (hereinafter, referred to as MAAs) are natural UV-A absorbing substances, which are known to be present in a wide variety of aquatic organisms such as coral, red algae, fish viscera and microalgae. Among others, shinorine is known to be the most effective natural UV-A absorbing substance. Chemical synthesis of an MAA has been tried; however it requires long and complicated processes (Patent Document 1). Also, production of an MAA by light irradiation of cyanobacteria has been tried; however, the production amount of MAA is extremely low (Non-Patent Document 1). Further, extraction of MAAs from natural products such as laver, algae and shellfish has been tried; however, a sufficient yield has not been obtained in these cases (Patent Document 2 and Non-Patent Documents 2 to 4). Furthermore, extraction and production from natural products are easily influenced by climate conditions and mostly unstable, and thus, it is difficult to obtain MAAs stably in a large amount. In the meantime, microbial biosyntheses of MAAs have been tried; however, these processes require disruption of cells and extraction with an organic solvent in order to obtain biosynthesized MAAs from the microbial cells (Patent Document 3 and Non-Patent Documents 5 and 6). Thus, the operation becomes complicated and an operation for removing debris derived from microbial cells is further required.